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  • Technical Review
  • Published:

Guide to optical spectroscopy of layered semiconductors

Nature Reviews Physics volume 3, pages 39–54 (2021)Cite this article

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Abstract

Potential applications in photonics and optoelectronics are based on our understanding of the light–matter interaction on an atomic monolayer scale. Atomically thin 2D transition metal dichalcogenides, such as MoS2 and WSe2, are model systems for layered semiconductors with a bandgap in the visible region of the optical spectrum. They can be assembled to form heterostructures and combine the unique properties of the constituent monolayers. In this Technical Review, we provide an introduction to optical spectroscopy for layered materials as a powerful, non-invasive tool to access details of the electronic band structure and crystal quality. We discuss the physical origin of the main absorption and emission features in the optical spectra and how they can be tuned. We explain key aspects of practical set-ups for performing experiments in different conditions and the important influence of the direct sample environment, such as substrates and encapsulation layers, on the emission and absorption mechanisms. A survey of optical techniques that probe the coupling between layers and analyse carrier polarization dynamics for spin- and valleytronics is provided.

Key points

  • Optical spectroscopy tools give access to details of the electronic band structure, crystal quality, crystal orientation, light–matter interaction and spin–valley polarization of 2D materials.

  • Key experimental parameters such as temperature, applied electric and magnetic fields, optical excitation power and the direct sample environment (such as substrate and encapsulation layers) strongly influence optical absorption and emission.

  • To achieve high spatial resolution, experiments on layered materials are carried out in optical microscopes. The high numerical aperture of the microscope objectives results in excitation and collection of light away from normal incidence, which gives access to information on optical transitions with different spatial orientations of the optical dipole.

  • In layered materials with strong excitonic effects, light–matter interaction is enhanced at specific energies. The emission as well as the absorption is therefore strongly energy-dependent, and light sources with tunable excitation provide flexibility for controlling optical absorption in the sample.

  • Using optical excitations with well-defined light polarization enables the excitation of carriers with specific spin and/or valley quantum numbers determined by the optical selection rules in the crystal. This reveals important information on the spin and valley dynamics in the material.

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Fig. 1: Excitons in transition metal dichalcogenide monolayers and heterobilayers for optical spectroscopy.
Fig. 2: Variation in photoluminescence response for different experimental conditions.
Fig. 3: Moiré interlayer excitons in heterobilayers.

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Acknowledgements

The authors acknowledge funding from ANR 2D-vdW-Spin, ANR VallEx, ANR MagicValley, ITN 4PHOTON Marie Sklodowska Curie Grant Agreement no. 721394 and the Institut Universitaire de France. The authors thank H. Tornatzky, D. Lagarde, A. Balocchi, N. Leisgang, H. Park and M. Glazov for discussions, and Y. Zhou for providing data from Zhou et al. Phys. Rev. Lett. 124, 027401 (2020).

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  1. Université de Toulouse, INSA-CNRS-UPS, LPCNO, Toulouse, France

    Shivangi Shree, Ioannis Paradisanos, Xavier Marie, Cedric Robert & Bernhard Urbaszek

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All authors contributed to the discussion of content and researched data for the article. S.S., I.P. and C.R. wrote the manuscript with critical input from X.M. and B.U.

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Glossary

Multilayers

Structures consisting of more than one layer.

Excitons

Coulomb-bound electron–hole pairs.

Bright and dark optical transitions

A spin- and dipole-allowed transition is ‘bright’ whereas a spin- and/or dipole-forbidden transition is ‘dark’.

Heterobilayers

Lateral or vertical heterojunctions formed by combining two different monolayers.

Moiré effects

Effects related to the interference pattern produced by the superposition of two slightly different lattice constants and/or twist angles.

Reconstructions

Spontaneous translational or angular rearrangements of atoms within multilayers, aiming for a lattice configuration with the lowest energy.

M 2 factor

Represents the degree of variation of a beam from an ideal Gaussian beam. This factor reflects how well a collimated laser beam can be focused to a small spot, or how well a divergent laser source can be collimated.

Quasiparticle bandgap

Bandgap of free electrons and holes; the exciton resonance energies lie in energy below the quasiparticle bandgap.

Homobilayers and homotrilayers

Stacking of two and three monolayers of the same material, respectively.

Four-wave mixing

Nonlinear effect arising from the third-order optical nonlinearity where one or two new wavelengths are produced by interactions between two or three wavelengths. Four-wave-mixing microspectroscopy accesses coherence and population dynamics of excitons.

Two-colour pump–probe experiments

Pump–probe experiments using two distinct laser beams where the wavelengths of the pump and probe beams are not identical.

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Shree, S., Paradisanos, I., Marie, X. et al. Guide to optical spectroscopy of layered semiconductors. Nat Rev Phys 3, 39–54 (2021). https://doi.org/10.1038/s42254-020-00259-1

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